The present invention relates to the "doping" of semiconductors by the implantation of ions into semiconductor substrates and, more particularly, to an improved plasma flood system for use in ion implantation equipment.
In the manufacture of semiconductors, it is common practice to implant specific species of ions at high doses into semiconductor substrates in the form of semiconductor wafers. The equipment utilized to perform such ion implantation usually comprises an ion source, a mass analyzer having an ion acceleration tube input and output to transport ions to a semiconductor wafer supported on a rotating and transversely moveable disk. In addition, the equipment includes a monitoring system for monitoring the ion beam current in order to control the dose of ions being implanted. The monitoring system commonly employs a Faraday "cage" before the wafer or either a Faraday "cage" or a magnetically suppressed beam stop after the wafer.
As the ions are implanted in the semiconductor, the surface of the semiconductor wafer becomes positively charged. If the surface charge reaches a value greater than the insulation breakdown voltage for an insulating film on the semiconductor wafer, the insulating film will break down. To prevent such an occurrence, it is common for ion implantation equipment to include an electron flood gun or similar device for directing onto the wafer surface a stream or "flood" of electrons which offsets and at least partially neutralizes the buildup of positive charge on the surface of the semiconductor wafer.
An example of such a combination is described in U.S. Pat. No. 5,089,710 issued Feb. 18, 1992 for "Ion Implantation Equipment". The patent describes equipment including a Faraday tube located adjacent a semiconductor wafer for receiving an ion beam. The Faraday tube is provided to monitor beam current and to prevent secondary electrons which are generated when the ions are implanted in the surface of the semiconductor wafer from escaping the Faraday system. The Faraday tube is positively biased relative to a plasma generation chamber mounted through a wall of the Faraday tube. The plasma generation chamber is placed at a position where a filament in the chamber does not face the semiconductor wafer. Argon gas is introduced into the plasma chamber and thermoelectrons emitted from the filament act on the gas to generate a plasma. Electrons at energies above 13 eV pass with the plasma through a small exit aperture in the plasma generation chamber into the ion beam in the Faraday tube. The Faraday tube is maintained at a positive potential and effects an extraction and acceleration of the electrons from the plasma generation chamber to higher energy levels. Such high energy electrons in striking the wall surface of the Faraday tube produce secondary electrons whose statistical energy distribution typically extends to well over 20 eV.
A serious shortcoming of the design described in the '710 patent is that the electron current flow must be adjusted extremely critically to prevent charge build-up on the wafer. In particular, if the electron current is adjusted slightly too high, the electrons have such a high energy level distribution that they will not be deflected from impinging the wafer, even if the wafer already has accumulated a negative charge. A further disadvantage of this design is that the positively biased Faraday rapidly attracts and absorbs low energy primary electrons and low energy secondary electrons produced at the surface of the semiconductor wafer which otherwise would be useful in neutralizing positive charging of the wafer surface.